In the purification of industrial and communal waster waters in mechanical-biological sewage treatment plants there also accrue besides the purified waste waters, which can be carried off without problems in the main canal or ditch relatively large amounts of sludge which likewise must be duly eliminated.
The sludge (fresh sludge) generally is deposited together from the primary sludge of the mechanical step of a sewage treatment plant (prelimnary sewage treatment) and the so-called surplus sludge which is derived from the biological step. Insofar as the waste water to be treated is additionally purified by way of a chemical precipitation, the so-called third purification step, there still accumulates a sludge with a predominantly inorganic character.
The sludge generally has a low dry material content and contains large amounts of microorganisms even of the pathogenic type, in the form of bacteria, viruses and grubs of intestinal parasites., A further criterion of sludges generally is a complete oxygen deficit.
Through anerobic bacteria activity, i.e., through bacteria activity in the absence of dissolved oxygen and without introduction of air, there occurs, especially with sludges (sewage sludges), very quickly the formation of extremely bad and partially toxic odors which create a great problem in the treatment and removal of the sludge both for the operational personnel of a sewage treatment plant and also for the neighborhood.
The process of anaerobic, biochemical bacterial activity is designated as "acid fermentation" and is a process which is extremely undesired in waste water technique and particularly in the treatment of sludge. In this process oxygen necessary for the bacterial respiration is split off from oxygen containing compounds, in a given case even from the water molecules, whereby gases such as carbon dioxide (CO.sub.2), hydrogen (H.sub.2), hydrogen sulfide (H.sub.2 S), and small amounts of methane (CH.sub.4) are formed. Furthermore there are formed through the bacterial activity otherwise unpleasant and ill-smelling sulfur and nitrogen containing odoriferous materials as e.g., mercaptans, thioethers or even ammonia. The splitting of fats contained in the waste water sludge by bacteria leads to the formation of organic acids such as acetic acid and butyric acid whereby the pH of the originally neutral sludge after a very short time is converted into the weakly acid range.
In the treatment and elimination of sludge, be it through disposal in agriculture, thickening by means of a thickner, dewatering by centrifuges or by thermal (drying, burning) or chemical treatment, it is anti-social to move, stir up or transport the sludge with the help of pumps or other aggregates.
Precisely in these necessary type of sludge treatment there are strongly emitted the previously mentioned ill-smelling and partially toxic odoriferous materials through which there occurs a large annoyance and danger to the health for the operating personnel and neighborhood of the sewage treatment plant.
However, while on the one hand uncontrolled, anerobic sludge decay in the form of the acid fermentation in the sludge collection hoppers of the preliminary sewage treatment and the thickening containers (preliminary thickners) is extremely undesired, on the other hand on the contrary there is consciously used the controlled, microbial decomposition of the organic contents of the sewage sludge in the so-called alkaline sludge decay, indeed it is required by an optimal pH and temperature control.
In this process the organic material is converted to methane (methane decay) with the help of specific groups of bacteria.
Since the sludge elimination in e.g., communal sewage treatment plants in nearly all cases previously were developed by way of the process of methane decay it was important therefore in many cases not to kill the microorganisms in the sludge, the sludge thus was not sterilized but only to check their vital processes to as great an extent as possible in the region of the sewage treatment plant in which the sludge is worked up, particularly in open systems.
However, should it not be decayed as, e.g. in storage in deposits, in transportation or in application to agriculture land, even then a nearly complete killing of all bacteria can take place since a deodorization is desired only for a specific time, namely for the time of further treatment.
Thus, e.g., for recovery of a sludge suitable for the treatment of fields there was described a combined treatment of this sludge through heat and agents splitting off chlorine, for total killing of the bacteria, see Johansson German OS No. 2460286.
This kind of sludge, however, can no longer be decomposed. Therefore it was essential for the art that there be a process in which a completely adequate deodorization occurs but not simultaneously restricting a complete killing of the bacteria.